Removal of humic substances by coagulation involves nano- and microparticle transport processes. The objective of this paper has been to describe the effects of polymer characteristics on the initial coagulation of nano-sized humic substances and on the aggregates’ ability to form larger flocs. The study offers a direct comparison of four different low molecular weight polycations, with charge densities ranging from 4.0 to 7.0 meq/g, as well as of a low and medium molecular weight cationic polyacrylamide with practically equal charge densities. The extent of coagulation of humic substances, determined as the percentage removal of humic substances after filtration through 0.1 μm, could, regardless of the polymer type, be explained by the amount of cationic charge equivalents added per mg TOC of humic substances. The optimal polymer dosage with respect to the extent of flocculation, determined as the percentage removal after filtration through 11 μm could not be explained by this, but the maximum extent of flocculation obtained with each polymer type increased with increasing polyelectrolyte charge density. However, the weak polycation chitosan showed a significantly higher maximum extent of flocculation than would be predicted from its charge density. Polyelectrolyte molecular weight did not show any significant effect on the coagulation of humic substances, nor did it increase the extent of floc separability at 11 μm.

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